1. Field of the Invention
The present invention relates to a surface emitting laser, a surface emitting laser array and an image formation apparatus.
2. Description of the Related Art
The vertical cavity surface emitting laser (VCSEL) is a laser that emits a laser light in a direction perpendicular to the in-plane direction of a semiconductor substrate. A surface emitting laser has excellent features such as that it provides a stable single mode as a longitudinal mode, that it has a lower threshold value compared to that of an edge emitting laser, and that it is easy to be formed into a two-dimensional array. Because of those features, there are expectations for surface emitting lasers to be used as a light source for optical communications and optical transmissions, and a light source for electrophotography.
In a surface emitting laser, the control of a transverse mode to be oscillated is a critical issue. Considering applications to communications etc., the transverse mode is required to be a single mode. Therefore, up to now, as disclosed in U.S. Pat. No. 5,493,577, the emitting region of an active layer is limited by providing a current confinement structure within the element through a selective oxidation, and at the same time, a single mode is ensured by forming a waveguide structure with the selectively oxidized portion.
However, a surface emitting laser having a structure as shown by U.S. Pat. No. 5,493,577 has a problem in that a high-order transverse mode becomes more likely to take place as described below. That is, in the surface emitting laser having a current confinement region described in U.S. Pat. No. 5,493,577, the current injected from an electrode disposed around a light emitting portion is confined to a smaller cross-section in a current confining portion as shown in the schematic diagram of FIG. 6A. Because of this, in the active layer as well, the current distribution becomes to have a peak in the vicinity corresponding to the peripheral portion within the current confinement region, as indicated by a solid line in FIG. 6B.
Since, due to such a current distribution, the gain of the active layer increases in the vicinity corresponding to the peripheral portion of the current confinement region as well, the gain of a high-order transverse mode inevitably increases and a high-order transverse mode is strongly excited.
Thus, increasing the injection current to obtain a high optical output power will result in that an oscillation of high-order transverse mode becomes more likely to take place.